STRUCTURAL COLOURS 173 



and refraction by minute structural details exhibited in the scales 

 or other cuticular parts. As Mason (1926) observes, whenever we 

 have practically colourless cuticle, in such a form as to present 

 many more or less unordered reflecting surfaces, whiteness is 

 produced. All whites in insects are ultimately explainable upon 

 a purely structural basis. Proof of this is afforded by the fact 

 that if the air in contact with the parts in question be replaced by 

 a colourless liquid, whose refractive index closely approximates to 

 that of chitin {i.e., n = 1-55 — 1-6), whiteness disappears ; after 

 removal of the liquid and drying the white appearance becomes 

 fully restored. In the white Pierine butterflies, Hopkins showed, 

 a number of years ago, that uric acid becomes stored in the wing 

 scales of certain of these insects, and he regarded the phenomenon 

 as an example of an excretory product functioning as a colour 

 producer. According to Mason, the white Pierine butterflies form 

 no exception to the rule that whiteness is a structural feature only. 

 Although uric acid is white, and might well function as a pigment 

 when finely divided, he states that it plays only a very minor 

 part in producing the white appearance of the butterfly scales. 

 He mentions that, after such solvents as dilute alkalies have 

 dissolved out the uric acid, the white remains undiminished after 

 the wings have been washed and dried. On the other hand, a white 

 Pierine wing becomes transparent when submerged in liquids of 

 the appropriate refractive index. He therefore concludes that, in 

 so far as whiteness is concerned, the presence or absence of uric 

 acid is of little importance. 



A variety of possible causes has been regarded as responsible 

 for the production of " metallic " or iridescent colours, and the 

 subject has been treated at length by Onslow (1921), and more 

 recently by Suffert (1924), and by Mason (1927). The theories 

 which have been advanced fall under four categories. 



1. Diffraction of light at the surface of a grooved structure or 

 " grating." 



2. Interference of light at the surfaces of single or multiple 

 " thin films." 



3. Scattering of shorter light- waves by minute particles smaller 

 in diameter than the waves themselves. 



